The nuclear fuel cycle is the series of industrial processes used to produce electricity from uranium in a nuclear reactor. The nuclear fuel cycle can be described as having three major parts: (1) the “front end” where uranium is mined and processed into fuel for use in a nuclear reactor, (2) the use of the fuel in a reactor, and (3) the “back end” where spent fuel is stored and eventually disposed or reprocessed (if the spent fuel is reprocessed, remaining wastes would be temporarily stored and eventually disposed).
The nuclear fuel cycle begins with the extraction of uranium from ores or other natural sources. Uranium provides the basic fissile material or “fuel” for nearly all nuclear reactors. Extracted uranium consists almost entirely of two isotopes of uranium atoms, mostly uranium-238 (U-238) (99.3%) together with a much smaller fraction (0.7%) of the fissionable isotope uranium-235 or “U-235.”
In its natural state, mined uranium is only weakly radioactive meaning that it can be handled without the need for radiation shielding. Before it can be used in a commercial reactor, natural uranium must be purified and enriched to boost the amount of fissionable U-235 present in the fuel. Most of the commercial nuclear power plants in operation today use fuel enriched to a U-235 concentration of anywhere from 3 to 5 percent—a typical figure for fuel used in commercial U.S. reactors is 4 percent.
The enriched uranium is cast into hard pellets and stacked inside long metal tubes or “cladding” to form nuclear fuel rods. The uranium in the pellets is not pure elemental uranium but rather uranium oxide. The fuel rods are bundled into nuclear fuel rod assemblies that are typically about 12 to about 14 feet long. The core of a typical light-water commercial nuclear power reactor in the U.S. contains roughly about 200 to about 500 nuclear fuel rod assemblies, totaling approximately 100 metric tons of uranium oxide.
Inside the reactor, the enriched uranium sustains a series of controlled nuclear reactions that collectively liberate substantial quantities of energy. The energy is converted to steam and used to drive turbines that generate electricity. Meanwhile, the fission process inside the reactor creates new elements or “fission products,” and gives rise to some heavier elements, collectively known as “transuranics,” which may take part in further reactions (among the most important is plutonium-239).
The preponderant reactor type currently used in the majority of commercial nuclear power plants is the light water reactor (LWR). There are also several other reactor types in commercial use such as the heavy water reactor (HWR), gas cooled reactor (GCR), boiling water cooled graphite moderated pressure tube type of reactor (RBMK), etc.
Nuclear fuel remains in a commercial power reactor for about four to six years, after which it can no longer efficiently produce energy and is considered used or spent. The spent fuel removed from a reactor is thermally hot and emits a great deal of radiation. Upon removal from the reactor, each spent nuclear fuel rod assembly emits enough radiation to deliver a fatal radiation dose in minutes to someone in the immediate vicinity who is not adequately shielded.
The spent nuclear fuel rod assemblies are transferred to a deep, water-filled pool and stored in a rack. Wet storage keeps the spent fuel cool and protects the workers from the radiation. Ideally, spent fuel is kept in the pool for at least five years, although spent fuel at many U.S. reactor sites has been in pool storage for several decades.
After the fuel has cooled sufficiently in wet storage, it can be transferred to dry storage. Dry storage systems generally consist of multiple nuclear fuel rod assemblies positioned in a fuel storage grid that is placed in a steel inner container and a concrete and steel outer container.
The hazards posed by spent fuel makes it difficult to transport. For this reason, government regulators require spent fuel to be shipped in containers or casks that shield and contain the radioactivity and dissipate the heat. In the U.S., spent fuel has typically been transported via truck or rail although other nations also use ships for spent fuel transport.
Spent fuel can be reprocessed to produce additional nuclear fuel. Even after commercial fuel is considered “spent,” it still contains unused uranium along with other re-usable elements such as plutonium which is generated within the fuel while it is in the reactor and fission products. Current reprocessing technologies separate the spent fuel into three components: uranium, plutonium (or a plutonium-uranium mix), and waste, which contains fission products and transuranic elements that are produced within the fuel. The plutonium is mixed with uranium and fabricated into new fuel while the fission products and other waste elements are packaged into a new form for disposal.
Regardless of whether spent fuel is reprocessed or directly disposed of, every approach to the nuclear fuel cycle requires disposal of spent fuel that assures the very long-term isolation of radioactive wastes from the environment. Many nations, including those engaged in reprocessing, are working to develop disposal facilities for spent fuel and/or high level waste, but no such facility has yet been put into operation. Every nation that is developing disposal capacity plans to use a deep, mined geologic repository for this purpose.
The lack of operational disposal facilities makes storage that much more important. Storage in some form, for some period of time, is an inevitable part of the nuclear fuel cycle. In the early days of the nuclear energy industry it was assumed that storage times for spent fuel would be relatively short—on the order of several years to a decade or two at most—before spent fuel would be sent either for reprocessing or final disposal.
The current reality is much different. Storage is not only playing a more prominent and protracted role in the nuclear fuel cycle than once expected, it is the only element of the back end of the fuel cycle that is currently being deployed on an operational scale in the U.S. In fact, much larger quantities of spent fuel are being stored for much longer periods of time than policy-makers envisioned or utility companies planned for when most of the current fleet of reactors were built.
The dominant form of storage for spent fuel at operating reactor sites is wet storage in pools. In some countries, pools are even used at consolidated storage facilities that are distant from the reactor sites. Pools are the de facto storage solution because they are essential to operating a nuclear power plant given the need to cool newly discharged spent fuel close to the reactor core. Once spent fuel is in the pool, it is easy and inexpensive to leave it there for long periods of time.
Storing spent fuel in pools presents a number of problems. One problem is the limited capacity of the pools. Over the years, the pools fill up until there is no more additional capacity. The operator of the reactor must then transfer some of the spent fuel to dry storage, which is an expensive and difficult operation.
Another problem is that spent fuel stored in pools is susceptible to natural disasters such as earthquakes. The earthquake may cause the pool to lose water and the spent nuclear fuel to meltdown. The Fukushima disaster in Japan is an example of a cooling pool losing water causing the spent fuel to overheat and meltdown.
Another problem is that spent fuel can begin to lose its structural integrity when stored for long periods of time in a pool. Once this happens, the structural integrity of the spent fuel must be restored, a process that requires considerable time and resources.
After an initial period of cooling in wet storage (generally at least five years), dry storage (in casks or vaults) is the preferred option for extended periods of storage (i.e., multiple decades up to 100 years or possibly more). Unlike wet storage systems, dry systems are cooled by the natural circulation of air and are less vulnerable to system failures and natural disasters.
The most common type of dry storage system is shown in FIG. 1. The system includes a canister 12 that encloses multiple spent nuclear fuel rod assemblies 10. The canister 12 is positioned inside a concrete structure or cask 14. The canister 12 is formed of ½ inch to ⅝ inch thick stainless steel or concrete and serves as the primary boundary to confine radioactive material.
The canister 12 can be oriented vertically or horizontally inside the cask 14. The cask 14 is a reinforced concrete structure that provides shielding from radiation and protects the canister 12. The cask 14 can be positioned in a vault 16 for long term storage as shown in FIG. 2.
Casks can be designed and licensed as single-purpose casks (storage only), dual-purpose casks (storage and transport), and multi-purpose casks (storage, transport, and disposal). Typically, the more uses the casks are licensed for, the more they cost.
Conventional cask systems present a number of problems. One is that many nuclear power plants require expensive and time-consuming upgrades to make it possible to handle and maneuver the casks while loading them with spent nuclear fuel assemblies. For example, many of these plants have to retrofit the pool area with a larger overhead crane to handle the tremendous load of the casks. These improvements can cost tens of millions of dollars, which tends to deter plant operators from moving spent fuel from wet storage to dry storage.
Another problem with conventional cask systems is that unless the cask is a multi-purpose cask, there is a good chance that the bare fuel assemblies will need to be handled again in order to transport and/or eventually dispose of the spent fuel. Handling bare fuel greatly increases the difficulty and cost required to transport and/or dispose of the spent fuel.
The current management strategy for spent nuclear fuel relies on dry storage to provide adequate capacity to allow continued operation of commercial nuclear plants. Utilities meet their interim storage needs on an individual basis with large-capacity, dry storage casks that are focused on meeting existing storage and transportation requirements because disposal requirements are not available.
The problem with this is that disposal of the large canisters currently used by the commercial nuclear power industry represents many significant engineering and scientific challenges. Additionally, the expanded use of high-burnup (>45 GWd/MTU) fuel increases licensing uncertainty associated with transporting existing spent nuclear fuel.
The problem is exacerbated by the uncertainty surrounding the requirements for the geological disposal repository. For example, several repositories under consideration are formed of materials (i.e., clay/shale, salt, and crystalline rock) that require limited canister/cask sizes due to thermal or physical constraints. This combined with the above discussion indicates that the canister/casks that end up satisfying the as yet unknown disposal requirements will likely be significantly different than what is being used for dry storage today.
This difference means that the existing canisters/casks will likely need to be repackaged in canisters/casks that satisfy future transportation and disposal requirements. Repackaging the spent nuclear fuel for the purpose of transportation and/or disposal, particularly following an extended storage period, creates radiological, operational, and financial liabilities and uncertainties and should be avoided or minimized.
Given the current status, the most imminent service needed worldwide for spent fuel management is the supply of sufficient and prolonged storage capacity that solves one or more of the problems identified above for the future spent fuel inventory arising from both the continued operation of nuclear power plants and from the removal of fuel in preparation for plant decommissioning.